Applied Geochemistry 36 (2013) 125–131
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Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
Competitive interaction between soil-derived humic acid and phosphate
on goethite
Zhiyou Fu a,b, Fengchang Wu a,b,⇑, Kang Song c, Ying Lin a,b, Yingchen Bai a,b, Yuanrong Zhu a,b, John P. Giesy d,e
a
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
State Environmental Protection Key Laboratory for Lake Pollution Control, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
c
College of Geographical Science, Nanjing Normal University, Jiangsu Key Laboratory of Environmental Change and Ecological Construction, Nanjing, China
d
Department of Veterinary Biomedical Sciences and Toxicology Centre, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
e
Zoology Department and Center for Integrative Toxicology, Michigan State University, East Lansing, MI 48824, USA
b
a r t i c l e
i n f o
Article history:
Received 23 July 2012
Accepted 21 May 2013
Available online 13 June 2013
Editorial handling by G.M. Filippelli
a b s t r a c t
In order to better understand the influence and mechanism of soil-derived humic acid (SHA) on adsorption of P onto particles in soils, the amounts of PO4 adsorbed by synthetic goethite (a-FeOOH) were determined at different concentrations of SHA, pH, ionic strength and order of addition of adsorbents. Addition
of SHA can significantly reduce the amount of PO4 adsorption as much as 27.8%. Both generated electrostatic field and competition for adsorption sites were responsible for the mechanism by which SHA inhibited adsorption of PO4 by goethite. This conclusion was supported by measurement of total organic C
(TOC), infrared spectral features and Zeta potential. Adsorption of PO4 onto goethite was inversely proportional to pH. Order of addition of PO4 and SHA can influence adsorption of PO4 as follows: prior addition of PO4 P simultaneous addition > prior addition of SHA. Iron and SHA apparently form complexes
due to prior addition of SHA. Observations made during this study emphasized that PO4 forms different
types of complexes on the surface of goethite at different pH, which dominated the interaction of SHA and
PO4 adsorption on goethite. Based on these observations, the possible modes of SHA inhibition of PO4
adsorption on goethite were proposed.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Phosphorus is an essential nutrient for plants and is also the
limiting elemental nutrient for some estuarine and coastal ecosystems and most freshwater ecosystems, where it is a major cause of
eutrophication of freshwater lakes (Carpenter et al., 1998). Sorption of PO4 on metal oxides and hydroxides in soils and sediments
influences mobility and bioavailability of PO4 in terrestrial and
aquatic systems. It is well known that oxides and hydroxides of
Fe have a strong affinity for PO4; Organic matter present in these
systems can interact with Fe oxyhydroxides, thereby changing
characteristics of sorption of PO4 on surfaces of Fe oxyhydroxides
(Tipping and Higgins, 1982; Borggaard, 1983; Antelo et al., 2005).
In particular, goethite (a-FeOOH) is a stable oxide of Fe, which is
one of the (hydr) oxide minerals with the greatest abundance in
soil and sediment. Bonding of PO4 by goethite has been found to
be stronger than by the other adsorbents such as Al oxide and fer-
⇑ Corresponding author at: State Key Laboratory of Environmental Criteria and
Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing
100012, China. Tel.: +86 10 84915312; fax: +86 10 84915277.
E-mail address: wufengchang@vip.skleg.cn (F. Wu).
0883-2927/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.apgeochem.2013.05.015
rihydrite (Borggaard et al., 2005). Surface sites of a-FeOOH are preponderantly occupied by PO4 even if it is present at small
concentrations in water; adsorption of PO4 on goethite is generally
due to surface complexation (Sigg and Stumm, 1981). Other studies have suggested that the reaction between PO4 and goethite includes surface complexation and ternary adsorption/surface
precipitation reaction (Li and Stanforth, 2000; Hongshao and Stanforth, 2001; Ler and Stanforth, 2003).
It has been postulated that interactions that occur between PO4
and natural organic matter are involved in geochemical processes
that occur in water, soil and sediment. Natural organic matter contains both poorly defined organic compounds and weak organic
acids with a defined chemical structure. The organic compounds
predominating in the natural organic matter of soils and water
are mainly composed of humic substances. Humic acids (HA) are
an important component of humic substances (Stevenson, 1994;
Antelo et al., 2007). Interaction between HA and PO4 has not been
well studied and previous studies have focused on weak organic
acids and PO4 (Geelhoed et al., 1998; He et al., 1999; Weng et al.,
2008). In fact, in comparison with weak organic acids, a large number of surface groups might be involved in coordination to the surface for HA, which suggests that HA can compete relatively
strongly with anions such as PO4 (Geelhoed et al., 1998). However,
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Z. Fu et al. / Applied Geochemistry 36 (2013) 125–131
few studies are available for HA, thus limited conclusive evidence
exists about effects of HA adsorption of PO4 on Fe oxyhydroxides.
HA is believed to adsorb onto Fe (hydr) oxides by displacement
of surface hydroxyl complexes (Tipping, 1981). Sedimentary and
soil-derived humic acid have been shown to compete with PO4
for sorption on goethite, thereby reducing adsorption of PO4 (Sibanda and Young, 1986; Hawke et al., 1989; Antelo et al., 2007).
However, the results of other studies have suggested that soil-derived humic acid (SHA) has a limited effect on adsorbed PO4. These
results were interpreted as a demonstration of considerably stronger affinity between PO4 and goethite than between HA and goethite (Borggaard et al., 2005). Alternatively, during early stages of
reaction, adsorption of P on poorly ordered Fe oxide is reduced
by competitive exchange reactions (Gerke, 1993). During the
course of reaction between humics and Fe oxide, adsorption of P
was shown to increase due to the inhibition of crystallization of
Fe oxide and complexing of Fe to organic matter (Gerke, 1993).
Therefore, the influence of HA on adsorption of PO4 on Fe-oxide
are still uncertain. These observations suggest that there are different adsorption mechanisms under different experimental conditions such as pH, order of addition of HA, crystallization of Fe
oxyhydroxides. Further elucidation of the interaction between
HA and PO4 on Fe oxyhydroxides is needed.
The aim of the present study was to help clarify the effects of
SHA on adsorption of PO4 by goethite, and to explore the relevant mechanism by which this occurs. This study is based on the
following null hypothesis: (1) SHA cannot affect adsorption of
PO4 on goethite; (2) SHA do not inhibit adsorption of PO4 on goethite by competing for the adsorption sites on the surface; (3)
Addition of SHA prior to PO4 addition will not increase inhibition
of SHA on PO4 adsorption by the surface of goethite.
2. Materials and methods
Substances Society (IHSS) (the SUVA254, SUVA280 and Caromatic of
IHSS HA are 3.04, 2.49 and 21.2 respectively).
2.3. Adsorption
Adsorption experiments were conducted in batch mode by adding 0.05 g of goethite in 50 mL polycarbonate centrifuge tubes and
was brought to a final volume to 25 mL. The suspensions contained
0–20 mg/L P (KH2PO4), 0.4 and 0.1 mg/mL SHA; 1.01 mg/mL
(0.01 M) KNO3 was used as a background solution. In kinetics studies, sorption of PO4 on goethite reached equilibrium within 50 h.
Therefore, all suspensions were adjusted to the expected pH and
shaken end-over-end (150–160 r/min) at room temperature
(22 °C) for 50 h with daily pH adjustments. After equilibrium,
suspensions were centrifuged at 5000 r/min for 15 min and supernatants were collected for measurements of pH, TOC and concentration of P. Concentrations of P in suspension were determined
by the molybdophosphate blue method (Agilent 8453, USA). Concentrations of SHA in solution were characterized by total organic
C (TOC) (Multi N/C 3100, Germany).
A portion of goethite samples before and after adsorption were
characterized by Fourier transform infrared spectrometry (FTIR,
PerkinElmer precisely LS55, USA). Electrophoretic mobility of
supernatants was measured by Zetasizer Nano ZS (Malvern Instruments, Malvern, UK). The order of addition of adsorbents was as
follows: simultaneous addition of SHA and PO4, prior addition of
PO4, prior addition of SHA. During prior addition of PO4, KH2PO4
was introduced into tubes and shaken for 50 h at room temperature (22 ± 1); and then SHA was added and shaken for another
50 h with daily pH adjustments. During the adjustment procedure,
centrifuge tubes were flushed with N2 to minimize the influence of
CO2. The method of prior addition of SHA was similar to that of
prior addition of PO4. All experiments were performed in triplicate
to obtain an average value.
2.1. Adsorbents
Goethite was prepared by use of previously described methods
(Schwertmann and Cornell, 2000; Gao and Mucci, 2001). Briefly, a
Fe(NO3)3 solution (100 mL 1 mol/L) was mixed with KOH solution
(180 mL 5 mol/L) in a vessel open to the ambient atmosphere and
diluted to 2 L with Nanopure™ water. That suspension was then
held in a closed polyethylene bottle at 70 °C for 60 h. After synthesizing the goethite, the precipitate was rinsed several times with
Nanopure™ water after centrifugation and decantation. The goethite slurry was then stored in Nanopure™ water in the refrigerator at 4 °C for 2 months to minimize changes in surface properties.
X-ray diffraction analysis (Cu Ka radiation) (Shimatz XD-D1, Japan)
of the precipitate are in good agreement with the standard mineral,
which is characterized by peaks at 21.188, 33.197, 36.626, 53.171
2h. D-spacings were 4.1896, 2.6962, 2.4514, 1.7211 respectively
(Fig. 1 for Supplementary data). The specific surface area of the
synthetic goethite was determined by the single-point N2-BET
method to be 30.32 m2/g (ASAP 2010M, USA).
2.2. Humic acid
Soil-derived humic acid was extracted, based on the procedure
recommended by the International Humic Substances Society
(Swift et al., 1996), from the soils from Jiu Mountain in Beijing, China. Soil humic acid were characterized by elemental analysis (Elementar vario macroEL, Germany), UV/visible adsorbance (Agilent
8453, USA), and solid state 13C nuclear magnetic resonance spectroscopy (Bruker Avance AV-400) (Table 1 for Supplementary
data). In general, SHA exhibited greater aromaticity, and a greater
degree of humification compared to the HA of International Humic
3. Results and discussion
3.1. Adsorption of phosphate during simultaneous addition of
adsorbents
Adsorption isotherms of PO4 in the presence and absence of
SHA, could be described by Langmuir isotherms, and reflected
the great affinity of the mineral surface for these types of substances. The data were fitted to a linear version of the following
equation:
Q ¼ abC eq =ð1 þ bC eq Þ
ð1Þ
where ‘‘Q’’ is adsorbed P (lg/g) at an equilibrium concentration of
Ceq (mg/L); ‘‘a’’ is the maximum P adsorption and ‘‘b’’ is a binding
Table 1
Langmuir phosphate adsorption maximum (a), binding constant (b) and determination coefficients (R2) for phosphate adsorption by goethite in absence and presence of
soil HA.
Adsorption
max, a (lg/g)
Binding constant,
b (L/mg)
R2
pH = 4.5
0 SHA
0.04 mg/mL SHA
0.1 mg/mL SHA
5478
3957
4256
3.57
2.53
1.07
0.959
0.922
0.878
pH = 7
0 SHA
0.04 mg/mL SHA
0.1 mg/mL SHA
4433
3371
3428
2.37
0.721
0.531
0.944
0.909
0.884
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Z. Fu et al. / Applied Geochemistry 36 (2013) 125–131
7000
(a)
(b)
5000
5000
P adsorbed (µg/g)
P adsorbed (µg/g)
6000
4000
3000
2000
0 mg/ml SHA
0.04 mg/ml SHA
0.1 mg/ml SHA
1000
4000
3000
2000
0 mg/ml SHA
0.04 mg/ml SHA
0.1 mg/ml SHA
1000
0
0
0
2
4
6
8
10
12
14
0
2
4
P solution concentration (mg/L)
6
8
10
12
14
16
18
P solution concentration (mg/L)
Fig. 1. Influence of SHA on phosphate adsorption on goethite (a – pH = 4.5 and b – pH = 7).
6.5
SHA adsorbed (TOC mg/g)
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
0.1 mg/ml SHA
0.04 mg/ml SHA
2.0
1.5
1.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
P adsorbed (mg/g)
Fig. 2. Relationship between adsorption of phosphate and SHA on goethite
(simultaneous addition, pH = 7).
1048
3120
2965
Absorbance
(affinity) constant under the given experimental conditions. Software of Origin 8.0 was used to calculate the fitting results.
Addition of SHA remarkably inhibited adsorption of PO4 onto
goethite at pH 4.5 and 7 (Table 1, Fig. 1). After addition of SHA,
the amount of PO4 adsorbed onto goethite and the binding constant both dropped. Binding constants between PO4 and goethite
were inversely proportional to concentration of SHA. These results
indicated that the presence of SHA resulted in weaker binding
affinity of PO4 on goethite. However, adsorption of PO4 on goethite
was not lower at greater concentrations of SHA (Fig. 1). This result
suggested that the effect of SHA on adsorption of PO4 on goethite
involved another mechanism besides competition for binding sites.
Previous studies have found divergent results on the effects of
SHA on adsorption of PO4 on goethite. Sibanda and Young (1986)
indicated that SHA remarkably inhibited adsorption of PO4 on goethite. In comparison, the presence of SHA at pH 5 has been shown
to have limited effect on adsorption of PO4 (Borggaard et al., 2005).
If PO4 and the SHA are adsorbed onto the same surface sites, the
results suggested a stronger affinity between PO4 and goethite
(Borggaard et al., 2005). It should be noted that Sibanda and Young
(1986) mixed goethite and SHA before addition of PO4. Prior addition of SHA might enhance the inhibition of PO4 adsorption.
Opposite trends were found between adsorption of PO4 and
SHA on goethite when SHA and PO4 were added simultaneously
(Fig. 2). Significantly, negative correlations were observed between
adsorption of PO4 and TOC (0.1 mg SHA/mL: r = 0.930,
p = 0.007; 0.04 mg/mL SHA: r = 0.892, p = 0.017). This relationship suggested that SHA might compete with PO4 for the same
adsorption sites on the surface of goethite.
The FTIR spectrum of goethite (Fig. 3) included a waveband at
900–4000 cm1, which is consistent with the range of typical soil
HAs (Senesi et al., 2003). Characteristic absorption peaks for goethite were observed at 3050–3500 cm1, which can be attributed
to the OAH stretching and H-bonded OH. Ratios of heights of peaks
in the IR spectra have been used to interpret IR spectra (Kang and
Xing, 2005). The ratios of the sum of peak heights (3050–3500)
after adsorption when compared to that of pure goethite, were increased in the following order: adsorption of PO4 (0.415) < adsorption of PO4 and 0.04 mg/mL SHA (0.694) < adsorption of PO4 and
0.1 mg/mL concentrations of SHA (0.910). These results indicated
fewer hydroxyl functional groups on the surface of goethite after
adsorption of PO4 and SHA. The increase of the peak characteristic
of hydroxyl for goethite on the addition of SHA and PO4 in comparison to that of only addition of PO4 can be ascribed to the intense
and broad peak of hydroxyl groups for SHA (Fig. 2 for Supplementary data). In fact, the intense peaks at 1386 cm1 and 2965 cm1
were found for goethite (Fig. 3) and SHA (Fig. 2 for Supplementary
data), can be ascribed to COO antisymmetric stretching and aliphatic CAH stretching, respectively, and are indicative of adsorption of SHA on goethite, Moreover, the 1660 cm1 peak (Fig. 3)
4000
5mg/L P
5mg/L P+0.04mg/ml SHA
5mg/L P+0.1mg/ml SHA
Absence of SHA and phosphate
1386
3500
3000
2500
2000
1500
1000
-1
Wave number (cm )
Fig. 3. The FTIR spectra of goethite samples before/after phosphate and SHA
adsorption (pH = 4.5).
can be attribed to C@O stretching of the amide groups (amide I
band), C@O of quinone and/or H-bonded conjugated ketones; Carboxyl functional group of SHA seems to have contributed to the
band intensity. The peak at 1090 cm1 was found only after addition of PO4, or PO4 and SHA adsorption simultaneously (Fig. 3);
SHA also exhibited the peak at 1021 cm1 (Fig. 2 in Supplementary
data). Intensities of these peaks can be attributed to the PAO bond
of PO4, which is consistent with results of previous works that the
characteristic P absorbance at 1100–1000 cm1 would provide a
simple and rapid diagnosis for P contained in humic substances
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Z. Fu et al. / Applied Geochemistry 36 (2013) 125–131
(He et al., 2006, 2011). These results suggested that one of the
mechanisms of interference of sorption of PO4 is that SHA competes for adsorption sites on the surface of goethite.
Zeta potential, which is directly related to electrophoretic
mobility, can also provide an estimate of the net charge of moving
particles. The net charge of the moving particle allows determination of the isoelectric point (IEP) of variable charge particles (Hunter, 1981; Rietra et al., 2000). The presence of either PO4 or SHA in
the goethite suspension leads to a substantial decrease in the Zeta
potential. Nevertheless, addition of greater concentrations of SHA
did not further reduce Zeta potential (Fig. 4). These results agree
well with data on adsorption of PO4, which also was not significantly less in the presence of greater concentrations of SHA. Addition of SHA resulted in a greater negative charge on the surface of
goethite, which generated a more negative electrostatic field. The
negatively charged electrostatic field is an unfavorable environment for adsorption of anions, which led to significantly less
adsorption of PO4 on goethite. The generated electrostatic field effects of SHA can also be considered as one of the important mechanisms for inhibition of adsorption of PO4 on goethite by SHA.
was inversely proportional to pH (Fig. 5b). At higher pH, there
was not only greater repulsion due to the negative charge on the
surface of goethite, but there was also a greater concentration
OH that could compete with PO4 for adsorption sites. Therefore,
it resulted in less adsorption of PO4 on goethite. In addition, PO4
forms a complex with hydroxyl groups on the surface of goethite
in either mono- or bidentate complexes. Studies have found both
forms on the surface. When monodentate surface complexes
formed, PO4 in the form of H2 PO
4 , one PO4 group only occupies
one adsorption site of hydroxyl (AOH). When bidentate complexes
formed, PO4 in the form of HPO2
4 , one PO4 group occupies two
adsorption sites of hydroxyl (AOH) (Eq. (2)). At higher pH, adsorption of PO4 on goethite was predominantly ‘‘bidentate complexes’’,
which resulted in less adsorption of PO4 (Hingston et al., 1974; Sigg
and Stumm, 1981; Geelhoed et al., 1998; Tejedor-Tejedor and
Anderson, 1990). Possible modes of interaction between PO4 and
SHA on goethite are presented in Eqs. (3) and (4), where X and Y
represent other components of SHA. Eqs. (3) and (4) included the
possible modes of interaction at pH = 4.5 (Eq. (3)) and at pH = 7
(Eq. (4)):
ð2Þ
ð3Þ
ð4Þ
When adsorption of PO4 was greater, there was virtually no release of either HA or FA into solution (Sibanda and Young, 1986).
Those researchers inferred that the unfavorable electrostatic field
generated around an adsorbed HA molecule is more important in
preventing adsorption of PO4. It has also been demonstrated that
the mechanism by which SHA affects adsorption of PO4 on mineral
oxides is a non-competitive adsorption (Appelt et al., 1975). Organic anions did not compete for or block adsorption sites for PO4 anions, because of the greater affinity of PO4 for adsorption sites in
soils derived from volcanic-ash (Appelt et al., 1975).
In contrast, at different concentration of PO4 (0.4 mg/L P), an inverse relationship between adsorption of PO4 and pH was observed
only in the presence of SHA. In the absence of SHA, adsorption of
PO4 on goethite was independent of pH (Fig. 5a). These results suggested that at lower concentrations of PO4, adsorption sites on goethite were sufficient to bind with PO4. Nevertheless, the soluble
PO4 concentrations measured under such conditions (absence of
SHA, 0.4 mg/L P) were close to the detection limit. Therefore, it cannot be ruled out that the detection limit might affect the adsorption results.
3.2. Effect of pH on adsorption SHA and PO4 on goethite
3.3. Effect of ionic strength on phosphate adsorption on goethite
Adsorption of PO4 on goethite at a pH of 4.5 was significantly
greater than that at pH 7 (p < 0.01) (Fig. 1). In either case the presence or absence of HA, adsorption of PO4 (5 mg P/L) on goethite
At pH 4.5, in the absence of SHA, PO4 adsorption decreased with
the decreasing concentration of KNO3 (Fig. 3a for Supplementary
data). According to Jones and O’Melia (2000) this effect of ionic
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Z. Fu et al. / Applied Geochemistry 36 (2013) 125–131
3.4. Effects of order of additions of PO4 and SHA on the surface of
goethite
25
20
Zeta potential (mV)
15
10
5
0
-5
-10
-15
-20
Goethite+water
5 mg/L P+0.1 mg/ml SHA
5 mg/L P+0.04 mg/ml SHA
-25
-30
2
3
4
5
6
7
8
9
10
11
pH
Fig. 4. Zeta potential of goethite in the presence and absence of SHA and phosphate.
100
100
90
90
80
80
70
P adsorbed (%)
P adsorbed (%)
strength is mainly due to changes in lateral electrostatic repulsion
among adsorbed molecules. Increased ionic strength can reduce
electrostatic repulsion between like-charged material (Jones and
O’Melia, 2000). At a lesser ionic strength there would be greater
lateral repulsion between adsorbed molecules which leads to reduced adsorption. In contrast, in the presence of SHA, no significant
effects on adsorption of PO4 on goethite were found as a function of
ionic strength (Fig. 3b; Fig. 3c for Supplementary data). Several
mechanisms can be invoked to explain the results. Greater lateral
repulsion between adsorbed SHA molecules takes place at lower
ionic strength. Alternatively, the expanded structure of adsorbed
SHA might have been produced at lower ionic strength, which resulted in SHA occupying a larger area of the oxide surface (Ghosh
and Schnitzer, 1980). Both of these factors may have resulted in
less adsorption of SHA such that there would be greater adsorption
of PO4.
At pH 7, adsorption of PO4 was directly proportional to the concentrations of KNO3. In the presence or absence of SHA, there were
no significant effects of ionic strength (Fig. 3d–f for Supplementary
data). This can be attributed to the fact that PO4 forms complexes
with the hydroxyl functional group to form more stable complexes
with bidentate complexes, due to the formation of six-membered
ring structures (Eq. (2)). Adsorption of SHA did not significantly
change trends in adsorption of PO4 on goethite surfaces as a function of ionic strength.
In some of the previous studies PO4 and SHA were not added
simultaneously. Thus, the effects of order of addition need to be
considered. At both pH 4.5 and 7, adsorption of PO4 was influenced
by the order in which reactants were added as follows: absence of
SHA > prior addition of PO4 > simultaneous addition > prior addition of SHA (Fig. 4 for Supplementary data). These results suggested that under the conditions of prior addition of SHA, SHA
occupied part of the adsorption sites and generated electrostatic
field, which resulted in less adsorption of PO4.
When goethite and SHA were combined prior to addition of PO4,
it has been shown that SHA significantly inhibited adsorption of
PO4 (Sibanda and Young, 1986). These results were consistent with
those presented here. In contrast, Antelo et al. (2007) indicated
that when SHA and PO4 were added simultaneously, adsorption
of PO4 was nearly the same as that of prior adsorption of HA, but
different from that of prior adsorption of PO4. Those authors suggested that the experiments with previous addition of PO4 did
not represent equilibrium situations (Antelo et al., 2007). However,
Borggaard et al. (2005) ruled out the influence of time to equilibrium as their results showed that adsorption onto goethite and
the adsorption maximum and binding constant were independent
of adsorption time between 3 and 28 days.
Adsorption of PO4 and SHA on goethite also exhibited the opposite trend to that observed for prior addition of PO4 (Fig. 6a). Negative correlations were observed between adsorption of PO4 and
TOC (0.1 mg SHA/mL: r = 0.569, p = 0.239; 0.04 mg/mL SHA:
r = 0.674, p = 0.142). These results indicated that PO4 and SHA
mutually inhibited adsorption on goethite. However, under conditions of prior addition of SHA, adsorption of SHA was no less at
greater adsorption of PO4 (Fig. 6b). No evident correlations were
observed between adsorption of PO4 and TOC (0.1 mg SHA/mL:
r = 0.065, p = 0.903; 0.04 mg/mL SHA: r = 0.221, p = 0.674). Furthermore, amounts of SHA adsorbed by goethite when it was added
prior to the other reactants, was greater than observed for other orders of addition. These observations suggested that the affinity between SHA and goethite was stronger than that between PO4 and
goethite.
The peaks of the FTIR spectra at 1048 cm1, 1393 cm1 and
2923 cm1 can be ascribed to CAO stretching of polysaccharidelike substances, COO antisymmetric stretching and aliphatic
CAH stretching, respectively. The intense peak of SHA at
1045 cm1 and 2923 cm1 are the characteristic peaks of SHA
(a)
60
50
30
(b)
60
50
Absence of SHA
0.1mg/ml SHA
0.04mg/ml SHA
40
Absence of SHA
0.1mg/ml SHA
0.04mg/ml SHA
40
70
30
20
2
3
4
5
6
pH
7
8
9
10
11
2
3
4
5
6
7
pH
Fig. 5. Relationship between pH and adsorption rate of phosphate (Left, 0.4 mg/L P. Right, 5 mg/L P).
8
9
10
11
12
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(a)
0.1 mg/ml SHA
0.04 mg/ml SHA
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
SHA adsorbed ( TOC mg/g)
Z. Fu et al. / Applied Geochemistry 36 (2013) 125–131
SHA adsorbed ( TOC mg/g)
130
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
(b)
0.1 mg/ml SHA
0.04 mg/ml SHA
0.0
P adsorbed (mg/g)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
P adsorbed (mg/g)
Fig. 6. Relationship between adsorption of phosphate and SHA on goethite at pH = 7 (a – prior addition of phosphate and b – prior addition of SHA).
(Fig. 2 in Supplementary data). However, the intense peak of SHA
at 1045 cm1, 1393 cm1 and 2923 cm1 in the FTIR spectrum,
were observed for goethite on the simultaneous addition of PO4
and SHA, rather than on the prior addition of SHA (Fig. 5 in Supplementary data). The sum of the peak heights (1045 cm1,
1393 cm1, 2923 cm1) of goethite decreased in the following order: simultaneous addition of PO4 and SHA (0.05079) > prior addition of SHA (0.01301). This was unexpected as the characteristic
peak of goethite should be more intense when adsorption of SHA
was greater. This might be attributed to the fact that, after the
adsorption of SHA, later adsorption of PO4 on goethite resulted in
the relative structural change of the SHA. Iron and SHA seem to
form complexes. Results of previous studies showed that Fe in humic complexes has a strong affinity for adsorption of P (Gerke and
Hermann, 1992; Gerke, 1993). Thus, there is also considerable
adsorption of PO4 on goethite despite prior addition of SHA, which
occupied a large number of adsorption sites. This result is consistent with the results that under conditions of prior addition of
SHA, adsorption of TOC was no less at the greater adsorption of
PO4 (Fig. 6b), which suggests that later addition of PO4 did not
markedly replace the SHA adsorbed on goethite.
3.5. Conclusions and environmental implications
This study demonstrated that SHA can significantly decrease
the adsorption of PO4 on goethite. The mechanism involved is competition for adsorption sites and development of a repulsive, negatively-charged electrostatic field. Iron and SHA seem to form
complexes on prior addition of SHA. The observations in this study
emphasized that PO4 forms different complexes with hydroxyl
functional group on the surface of goethite at different pH, which
dominated the interaction of SHA and PO4 adsorption on goethite.
In natural systems, natural organic matter, Fe minerals, and
phosphates co-occur in aquatic and terrestrial environments. Previous studies have suggested that a more realistic model of a natural water has to consider the adsorption of organic matter when
studying the adsorption of anions (including PO3
4 ) on goethite
(Sigg and Stumm, 1981). The study indicated that addition of soil
SHA can significantly reduce adsorption of PO4; which suggested
that organic matter can have important implications for bioavailability of PO4 in soils, and pollution of PO4 in lakes and rivers. In
addition, the least adsorption of PO4 was found for prior addition
of SHA; This may suggest that the exogenous PO4 will be less affected by Fe oxyhydroxides in lakes. The impact of natural organic
matter on the environmental behavior of PO4 may involve the geochemical processes of PO4 in soil and sediment. Further research
will be required to determine the processes and mechanisms.
Acknowledgements
This study was financed by the National Basic Research Program of China: ‘‘Water environmental quality evolution and water
quality criteria in lakes’’ (2008CB418200) and by the Natural Science Foundation of China (40973090, 41003047, 41130743,
41261140337).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/
j.apgeochem.2013.05.015.
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